A cast iron rocker arm used in an internal combustion engine is subjected to relatively high pressure, high speed rubbing against another cooperative component such as a cam lobe and/or valve stem. In some situations, the rocker arm wears rapidly as a result of friction and imperfect lubrication at the interface (contacting surfaces) between the rocker arm and the cooperative component.
One attempt at reducing wear of steel rocker arms and other wear-prone components has involved carburizing and/or nitriding to generate hard surfaces more resistant to wear. However, these surface hardening techniques add to the cost of the steel component and have not proven adequate in certain service applications.
An attempt at reducing wear of cast iron rocker arms has involved incorporating one or more metal chills in the casting mold at local regions corresponding to wear-prone areas of the casting to be formed. This technique has been used to cast rocker arms of a low alloy gray cast iron which is prone to form graphite in the microstructure depending upon the rate of cooling. The metal chill(s) accelerate the cooling rate and thus solidification of the iron at these local regions to essentially avoid formation of graphite and instead form a more wear resistant microstructure of iron carbides at the local regions. However, rocker arms cast in this manner will exhibit a complex microstructure having the carbidic constituent at the local, "chill-cast" regions and a graphitic constituent at other regions of the rocker arm.
Moreover, use of metal chill(s) in the casting of rocker arms not only adds to the cost of the final product but also results in dimensional variations that oftentimes necessitate subsequent extensive machining of the cast rocker arm to final tolerances.
Still another attempt at reducing wear of cast iron rocker arms has involved precision investment gravity casting of Cr-Ni alloy cast iron in preheated molds (e.g., 1800.degree. F. shell molds). The Cr-Ni alloy cast iron develops an as-cast microstructure having desired carbide constituent(s) (e.g., iron carbides, chromium carbides, etc.) in the matrix upon solidification. Representative of these Cr-Ni alloy cast irons is the commercially available Nihard.RTM. cast iron available from International Nickel Company and having a nominal composition, in weight percent (w/o), of 3.0 w/o C, 0.60 w/o Si, 0.60 w/o Mn, 4.5 w/o Ni, 3.0 w/o Cr and balance Fe. These Cr-Ni alloy cast irons are very expensive and add to the cost of the cast rocker arms.
It is an object of the present invention to provide an economical cast iron machine component or element, such as especially a rocker arm, resistant to wear in the as-cast condition without the need for subsequent surface hardening treatments.
It is another object of the present invention to provide an economical cast iron machine component or element, such as especially a rocker arm, resistant to wear in the as-cast condition without the need for incorporating metal chills in the casting mold.
It is another object of the present invention to provide an economical low alloy or high alloy cast iron machine component or element, such as especially a rocker arm, resistant to wear in the as-cast condition.
It is still another object of the present invention to provide an economical method of casting a wear resistant, cast iron machine component or element, such as especially a rocker arm, that eliminates the need for metal chill(s) in the casting mold and subsequent (post-cast) surface hardening treatments.